Much of the world's metallic aluminum is obtained from bauxite ores, which are composed essentially of aluminum hydrates, either gibbsite or boehmite. The United States has limited deposits of bauxite so that the development of alternative aluminum sources in this country is desirable to reduce any dependence upon foreign sources of the metal.
Many clays contain as much as 30-40% alumina and consequently are potential sources of alumina, but the known processes for recovering alumina from clays are usually of marginal economy. The cost of obtaining aluminum from local sources may, in some cases, not be competitive with the cost of imported material. To be economic, an alumina plant must have a very large capacity and be supported by an adequate supply of ore for processing. The process used must minimize energy consumption, maximize alumina yield and recycle reagents to reduce the cost of raw materials. These plants are very capital intensive and for this and other reasons mentioned above, it is highly desirable to reduce the cost of processing the ore.
In the extraction of alumina from alumino silicates such as clays, it is especially critical that the process be energy efficient; otherwise it is not economical to use a clay as the source material.
The extraction process usually involves the preparation of an intermediate aluminum compound from which alumina can be generated. This intermediate is usually the key to the process, since a number of characteristics of the process intimately associated with the intermediate determine the viability of the process. These characteristics include: the yield of alumina; the energy requirements; the ease of separating impurities; and the ability to recycle reagents for use in other steps of the process. In some processes, the intermediate is produced in a physical form which is difficult to handle and separate; in others it is necessary to use substantial amounts of energy to concentrate the intermediate by evaporation. In still others, it is difficult to separate the intermediate from impurities such as sodium and potassium and recover in the intermediate an acceptably large proportion of the aluminum which was present in the raw materials.
Processes for the extraction of alumina from alumino silicates, such as clays, are known. Many of these processes involve leaching the aluminum content from the raw material with an aqueous solution of a mineral acid, separating the resultant soluble aluminum compound from the insoluble silicate solids and reacting the compound to form an intermediate which can be converted into cell grade alumina. At some stage in the process, impurities such as sodium, potassium and iron must be separated from the alumina.
The three common mineral acids; hydrochloric, nitric and sulfuric acids have been used as leach acids. Hydrochloric acid has been used with some success to produce the chloride intermediate by crystallization but the corrosive nature of this acid requires rather exotic, and consequently expensive, plant construction materials. Sulfuric acid has been used to leach aluminum as aluminum sulfate into solution, but these processes have generally not been sufficiently economic. In sulfuric acid processes it is known to convert the sulfate under elevated temperature and pressure into an alunite intermediate, analogous to the naturally occuring mineral alunite, KAl.sub.3 (SO.sub.2).sub.4.(OH).sub.6. In U.S. Bureau of Mines Report of Investigation 7162, 1968, synthetic solutions comprising sodium sulfate, sulfuric acid and aluminum sulfate, and representing the leach solution of a low grade aluminum silicate, were autoclaved to produce a natroalunite precipitate which was washed and subsequently calcined to alumina. However, the alunite in this process suffers from the disadvantage of being a sodium compound. Since the product alumina must not contain a significant amount of sodium it is undesirable to use a sodium intermediate such as natroalunite. This increases the overall cost of the process and it is more difficult to remove the relatively large quantity of sodium derived from the sodium intermediate than to remove small quantities of sodium which may be present in the raw material.
It is also known to convert the sulfate to a basic aluminum sulfate, hydrogen alunite, which is similar to natural alunite but has hydronium ions substituted for potassium ions. In Extractive Metallurgy of Aluminum, Volume 1, pp. 305-332, 1963, aluminum sulfate solutions derived from leaching low grade bauxites with sulfuric acid solution were autoclaved at 180.degree. to 280.degree. C. to precipitate basic aluminum sulfate crystals, 3Al.sub.2 O.sub.3.4SO.sub.3.9H.sub.2 O. In U.S. Pat. No. 4,244,928, the ore is subjected to a two stage sulfuric acid leach, the first under pressure and a lower acid concentration than the second. The sulfate is hydrolyzed under pressure to basic aluminum sulfate. While these processes avoid the contamination introduced by using alkali metal intermediates, hydrogen alunite intermediate processes require high temperatures and pressures and the yield of alunite is too low.
Other known processes use sulfuric acid or a sulfate to leach the aluminous raw material and form an alum intermediate. In U.S. Bureau of Mines Report of Investigations 6290, 1963, clay is leached with a solution of sulfuric acid and potassium sulfate to produce normal alum, K.sub.2 SO.sub.4.Al.sub.2 O.sub.3.3SO.sub.3.24H.sub.2 O, which is crystallized, separated and autoclaved to produce basic alum, K.sub.2 SO.sub.4.3Al.sub.2 O.sub.3.4SO.sub.3.9H.sub.2 O. This is said to avoid processing difficulties when calcining normal alum to form alumina. However, the process also has the disadvantage mentioned above of introducing alkali metal ion contaminant into the process.
U.S. Bureau of Mines Report of Investigations 6573, 1965, discloses a process wherein clay is calcined with ammonium bisulfate or baked with ammonium sulfate to form ammonium alum which is twice crystallized and redissolved before being treated with ammonium hydroxide to produce alumina trihydrate.
U.S. Pat. Nos. 1,677,157 and 1,752,599, also disclose processes where treatment with ammonium sulfate forms ammonium alum which is separated by crystallization. In the former, the alum is reacted with ammonia liberated during alum formation to produce ammonium sulfate for recycling and hydrated alumina. In the latter, initial sulfatization with ammonium sulfate is carried out in an oxygen-free atmosphere containing gases produced by dissociation of the sulfate. The alum is converted to aluminum sulfate in an atmosphere which absorbs product ammonia as a reusable ammonium salt and the liquor from alum formation is reacted with ammonia to recover additional aluminum as the hydroxide.
All these alum processes suffer from the disadvantage of requiring crystallization of the alum intermediate. Aluminum sulfate solutions, when crystallized, form various hydrated alum compounds such as Al.sub.2 (SO.sub.4).sub.3.18H.sub.2 O aluminum sulfate (alum); NH.sub.4 Al(SO.sub.4).sub.2.12H.sub.2 O ammonium alum; and KAl(SO.sub.4).sub.2.12H.sub.2 O potassium alum. These alum crystallization processes require considerable energy to recover and calcine the intermediate and to regenerate acid for leaching and they are therefore costly.
There is a need, therefore, for a process for extracting aluminum from low grade materials such as clays which is economic and involves convenient, efficient, lower energy consuming process steps which provide easily handleable intermediates recovering a high proportion of the aluminum in the raw material without alkali or other major metal contamination.